Vapor-liquid contacting apparatus



Sept- 25, 1962 A. C. ELD ETAL VAPOR-LIQUID CONTACTING APPARATUS FiledApril 8, 1959 mmmmz jlllll will] lllllll flllll MMA A n M ATA L w tlINVENTORS 4A $L C. 540

BYW/AA/AMZ (ERR/4N AIZGQ/VEY United States Patent Ofiflce 3,055,646Patented Sept. 25, 1962 3,055,646 VAPOR-LIQUID CONTACTING APPARATUSAksel C. Eld, Mount Lebanon, and William R. Lehrian, Penn Hills, Pa.,assignors to Gulf Oil Corporation, Pittsburgh, Pa., a corporation ofPennsylvania Filed Apr. 8, 1959, Ser. No. 804,917 2 Claims. (Cl. 261114)This invention relates to improvements in trays for vapor-liquidcontacting towers and more particularly to an improved bubble capstructure for such trays.

Bubble tray towers have great importance in vaporliquid contactingprocedures such as fractional distillation of fluid mixtures, absorptionof gases in liquids, and stripping of volatile components from liquids.The tower is used for bringing a stream of vapor and a stream of liquidinto intimate countercurrent contact.

Bubble tray towers are provided with a series of horizontal trays,usually equally spaced apart in the vertical tower. Liquid flowsdownwardly from tray to tray and flows horizontally across each tray, aliquid level being maintained on each tray by overflow weirs. Vaporrises through openings or vapor passages in each tray and is intimatelymixed with the liquid flowing across the tray. In the conventionalbubble cap tray, a tubular chimney surrounds each opening or vaporpassagev and a stationary bell-shaped cap surmounts each chimney. Thevapor rises from below the tray through the chimney, is diverteddownwardly by the cap and bubbles out through slots in the cap or underthe edges of the cap.

When the ratio of liquid to vapor is not excessive the conventionalbubble caps function reasonably well. However, in some operations theyhave serious disadvantages. For example, when the liquid flow greatlyexceeds the vapor flow, as may occur in high pressure fractionation ofhydrocarbon mixtures, there is a serious danger of liquid dumping whichis the flow of liquid under the bubble caps and down the column throughthe vapor chimneys. When this occurs proper vapor-liquid contact is notobtained.

Bubble trays have been designed with the specific purpose of avoidingliquid dumping. One such tray employs a floating plate cap over eachhole in the tray. The periphery of the cap rests on the tray floor andforms a liquid seal. This type of cap is simply a freely floating disc,loosely enclosed within an overhead framework that limits its upwardmovement. For any particular cap, when the vapor velocity issufiiciently high, the cap is raised from the tray floor and vapor canbubble through the liquid on the tray. When the vapor velocity is low,the cap remains seated on the tray floor covering the hole in the trayand sealing against the flow of liquid downwardly through the vaporpassage. 7

The conventional floating plate cap type of bubble tray has to someextent reduced the problem of liquid dumping in fractionating towers.However, it has not been entirely successful in this respect and,furthermore, has other disadvantages. In the conventional structure eachhole or vapor passage in the tray is covered by a single floating cap,the cap being only slightly larger in area than the hole. When the capis lifted all of the vapor flowing from under the cap flows through theone large hole. Consequently, there is a tendency to form largebubbles.v Furthermore, the cap is easily tiltable and all of the vapormay escape under one side of the tilted cap. As a result of the largebubbles and the poor distribution of bubbles about the rim of the cap,intimate contacting of all portions of the vapor and liquid is notobtained.

The conventional floating plate cap has another disadvantage. As we havesaid, this type of cap is enclosed by an overhead framework. Each trayhas a large number of these frameworks extending upwardly from itssurface and they provide a substantial resistance to liquid flow acrossthe tray. Because of the flow resistance there is a substantialdifference in liquid level or head across the tray, the liquid levelbeing much higher on the inlet side of the tray than on the outlet side.A marked difierence in liquid head across the tray is undesirablebecause, unless other steps are taken to prevent it, the vapor will risemore readily on the side of the tray with low liquid level and theentire tray surface will not be used efliciently. Another disadvantageof the described type of tray is the difficulty of cleaning the surfaceof the tray. The overhead frameworks surmounting the vapor passagesprovide collecting places for tarry deposits and the irregular surfaceof the tray is diflicult to clean when the tower is shut down forperiodic cleaning.

Our present invention provides an improved bubble tray structure whichavoids or reduces the problems of liquid dumping encountered withconventional bell cap and chimney bubble trays and the problems of poorliquid-vapor contact, excessive resistance to liquid flow and difficultyof cleaning that are encountered with the conventional floating platecap type of tray. In general, the apparatus of our invention comprises avapor-liquid contacting tray having a plurality of openings which serveas vapor passages. Each opening is provided with an upwardly movable orvertically displaceable bubble cap assembly comprising a disc-like capsurmounting the opening in the tray. The cap is provided with adepending, vertical cylinder which hangs through the opening in thetray. The cylinder has apertures, such as vertical slots, spaced aboutits circumference, and the lower end or base of the cylinder is open.The lower rim of the cylinder is provided with an outturned lateralextension or flange which is spaced a short distance below the tray whenthe cap rests on the tray floor and which limits the upward movement ofthe cap.

We will describe our invention in more detail by reference to thedrawings of which:

FIGURE 1 is a schematic sectional view of a portion of a fractionatingcolumn providing with trays of our invention;

FIGURE 2 is a plan view, partly broken away, of one modification of abubble cap assembly such as shown on the trays of FIGURE 1;

FIGURE 3 is a sectional view in elevation of the bubble cap assemblyalong line IIIIII of FIGURE 2, the assembly being in its lowermostposition;

FIGURE 4 is a sectional view in elevation of another modification of thebubble cap assembly, the assembly being raised to its uppermostposition;

FIGURE 5 is a sectional plan view of the bubble cap assembly of FIGURE 4taken along line VV of FIG- URE 4; and FIGURE 6 is a view of astamped-out piece of sheet metal which can be used to form thecylindrical member of the. bubble cap assembly.

In FIGURE 1 a portion, of the interior of the fractionating tower 10 isshown diagrammatically. The vertical tower is provided with a series ofhorizontal trays, such as tray 12, which are usually spaced apartuniformly throughout the tower. Tray 12 has an outlet weir 14 extendingacross one side of the tray and an inlet weir 15 extending across theother side, although an inlet Weir is not always essential formaintaining a liquid level on the tray. On the outlet side a downflowapron 16 extends below the tray and the outlet weir 14. The apron 16 andthe wall of column 10 form a passage for the flow of liquid to the traybelow. Likewise, liquid flows from the tray above tray '12 through thepassage formed by apron flows over weir15 across tray 12 and over weir14. In

this manner a level of flowing liquid is maintained across tray 12 abovethe bubble caps.

Tray 12 has openings or vapor passages 18 arranged in rows and eachvapor passage is provided with a bubble cap assembly 19. The structureof these bubble cap assemblies is shown in detail in FIGURES 2-5.Referring to FIGURES 2 and 3, the assembly 19 comprises a disclike,generally horizontal cap 20 which surmounts each opening 18 in the tray.Cap 20 can be a flat disc but, preferably, as shown in FIGURE 3, theunderside of the cap 20 is concave. The rim 21 of the cap rests on thesurface of tray 12 when vapor is not passing upwardly through openings18 and entirely surrounds the opening covered by the cap. The rim of thecap thus forms a seal against the flow of liquid into openings 18.

The bubble cap assembly 19 further comprises a depending hollowcylindrical member 23. The upper rim 24 of the cylindrical member 23 isattached in any suitable manner to the underside of cap 20 such as bythe use of lugs 25 which pass through slots in the cap 20 and are peenedover so that the longitudinal axis of the cylindrical member passessubstantially through the center of the generally circular cap 20.

Along its lower rim the cylindrical member is provided with a flange orlateral member 26. The flange member 26 extends radially from thecylindrical member 23 in a plane generally parallel to that of the cap20. Usually, the flange member will extend about the same distance fromthe cylindrical member 23 as the periphery of cap 20, although it canextend somewhat more or less than cap 20. The essential feature offlange 26 is that its dimensions are such that the flange will limit theupward movement of cap 20 by abutting against the underside of tray 12when the cap is raised to its uppermost position, as in FIGURE 4. Thebubble cap assembly 19 is free to move vertically a distance equal tothe distance between the bottom of the tray floor 12 and the uppersurface of the flange member 26. Its lateral movement is limited by thesides of the cylindrical member 23.

To reduce the possibility of the flange member 26 sticking to theunderside of tray 12 when the bubble cap assembly is raised, the uppersurface of the flange member or the under surface of the tray in thearea of contact can be provided with raised portions of small surfacearea to prevent flat surface contact of the tray and flange member.Thus, as shown in FIGURE 3, small raised points 27 can be cast orstamped in the upper surface of the flange member 26.

The cylindrical member 23 is provided with a plurality of vertical slots28. These slots are usually uniformly spaced about the circumference ofthe cylindrical member 23 and extend from about the lower rim to theupper rim of member 23. Preferably, the slots are trapezoidal as shownin FIGURE 3 and have an enlarged opening 30 at the upper ends thereofbut can be of any shape that will form small streams of vapor, e.g.,triangular, rectangular, oval, etc.

We will describe the functioning of our bubble cap trays in fractionaldistillation of fluid mixtures by reference to one particular tray andbubble cap assembly as shown in the drawings. As we have indicated,liquid flows downwardly to tray 12 and over the inlet weir '15. A liquidlevel is formed between the weirs 14 and 15. The liquid moveshorizontally across tray 12, flowing over weir 14 and then down to thenext lower tray. Vapor formed on the next lower tray rises upwardlythrough the open end 3-1 of each bubble cap assembly and in small partthrough the annular space between the outer circumference of cylindricalmember 23 and the tray opening 18. If the vapor velocity is sufficientlyhigh, the bubble cap assembly is lifted from its position of rest ontray 12 as shown in FIGURE 3. The vapor passes through the open end 31of the assembly, passes through the slots 28 and bubbles upwardlythrough the liquid which flows across tray 12. Since the cylindricalmember 23 is provided with a plurality of slots 28, the vapor is dividedinto small portions flowing through the different openings. As a result,the vapor passes under the rim of cap 20 in small bubbles in reasonablyuniform distribution around the rim.

If the vapor velocity is not sulficient to force vapor upwardly throughthe openings in the tray, the bubble cap assembly 19 remains seated onthe floor of the tray as shown in FIGURE 3. The rim 21 resting on thetray floor forms a liquid seal and prevents liquid from flowing underthe cap and into the openings 13. Consequently, liquid dumping does notoccur.

If the vapor velocity is high, the bubble cap assembly will be raised toits highest position, as shown in FIG URE 4. It can also function inpositions between those of FIGURES 3 and 4. A plurality of vaporpassages is provided by the slots in the cylindrical member 23 and bythe generally annular space between the bubble cap assembly and thecircumference of tray opening 18. The vapor rising through opening 18 isthus divided into small portions. The formation of small bubbles ofvapor is promoted and intimate vapor-liquid contact is made possible.This is further favored by the fact that cap 20 tends to remainhorizontal when it rises. The hanging cylindrical member and itsattached flange member serve as a means to stabilize the cap againsttilting. This hanging weight gives the cap a low center of gravity andthe cap cannot be easily tilted to release large bubbles of vapor or topermit the leakage of liquid under the tilted rim of the cap.

The division of the vapor stream into a plurality of small streams canbe further promoted when the valve is in a partially open position byproviding the flange member 26 with a notched or serrated rim as shownin FIGURE 2. The notches can be short notches as shown in FIGURE 2 orthey can be deep notches extending all the way from the rim of theflange member to the cylindrical member 23. Deep notches will providevapor passages through the generally annular space between thecircumference of the opening 18 and the outer surface of the cylindricalmember 23 even when the bubble cap assembly is in the fully raisedposition. Being uniformly spaced about the assembly, these notches willfurther aid in providing uniform distribution of small streams of vaporfor contact with the liquid.

Within the scope of the invention modifications can be made in thestructure shown in the drawing. We have already indicated that the cap20 can be a flat disc or a concave-convex disc as shown in FIGURE 3. Theconcave-convex disc is preferred, because it aids in providing stabilityfor the cap when lifted by upflowing vapor. A generally circular shapefor the cap is preferred but other shapes are suitable. If desired, thecap can have a notched or serrated rim to improve vapor distribution butthe notches must not be so deep as to interfere with the liquid sealingaction of the cap when it rests on the tray floor as shown in FIGURE 3.

Although a concave-convex structure of the cap is advantageous, agenerally flattened cap as shown in the drawings is preferred becausesuch caps provide the least resistance to liquid flow across the trayand also facilitate cleaning. As we have indicated, one great advantageof our novel apparatus is that the tray surface is substantiallyunobstructed. It is possible for a workman to enter a tower providedwith our trays and to move about freely on the trays while cleaning thetrays or making any necessary repairs.

Another modification of our valve assembly is shown in FIGURES 4 and 5.The bubble cap assembly 19 comprises a cap 32 having a concavo-convexcentral portion and a flat rim 34. The rim 21 of cap 20 can rest flat onthe tray floor 12 as shown in FIGURES 2 and 3. However, if large areasof the cap rim and tray floor are in flat contact, the cap may stick tothe tray and require a very high vapor velocity to lift the cap from thetray initially. Therefore, in the preferred form of our bubble capassembly the lower surface of the cap rim or the upper surface of thetray beneath the cap is provided with a raised area of small surfacearea that contacts the opposite surface of the tray or cap. Forinstance, as shown in FIGURE 4, the underside of cap rim 34 of cap 32can be provided with a thin annular ridge 36. The cap rim 34 will thencontact the tray floor 12 only along this thin ridge 36 and the chancesof the cap 32 adhering unduly to the tray floor 12 will be reduced. Acontinuous annular ridge 36 on the rim 34 or tray 12 (not shown) willprovide about the same sealing action as flat surfaces of rim and tray.However, with little or no loss in sealing effect, the underside of thecap rim 34 or the upper surface of the tray 12 can have, instead of acontinuous ridge, a plurality of small raised points which will preventflat surface contact of the rim 34 and tray floor 12.

Numeral 38 of FIGURES 4 and refers to a cylindrical member which dependsfrom cap 32 and is attached in any suitable manner to the underside ofcap 32, such as by lugs 40, in a manner similar to that described forthe attachment of cylindrical member 23 to cap in FIGURES 2 and 3 above.The cylindrical member 38 is provided with a plurality of vertical slots42 which are generally uniformly spaced about the circumference of thecylindrical member 38.

In FIGURE 2, the flange member 26 was illustrated as a serrated ornotched continuous flange. Other structures performing its function canalso be employed. The essential feature is that the flange or lateralmember extends from the cylindrical member sufliciently to abut againstthe underside of tray 12 when the cap assembly is in the raised positionof FIGURE 4 and thus limits the upward movement of the assembly.However, the lateral member need not be a continuous flange as shown butcan comprise two or more, or preferably three or more, lateralextensions which will abut against the underside of the tray when thecap assembly is raised. FIGURES 4 and 5 illustrate a modification of theapparatus in which the flange or lateral member is not continuous. Asshown in FIGURES 4 and 5, four lateral extensions 44, extend from thelower rim of cylindrical member 38. These extensions 44, can be simplyoutturned lugs or ears integral with or attached to the lower rim of thecylindrical member. Three or more of such extensions will usuallyperform satisfactorily the function of the flange member of limiting theupward movement of the bubble cap assembly.

FIGURE 4 illustrates another modified feature of the lateral extensions44. As shown in FIGURE 4, the lateral extensions 44 are curved outwardlyfrom the cylindrical member 38 instead of having the perpendicularrelationship of the flange member 26 and cylindrical member 23 as shownin FIGURE 3. The curved structure of FIGURE 4 has the advantage that theupper surface of the flange member 44 and the undersurface of the trayfloor do not come into flat contact. possibility of the two surfacessticking together and cansing the cap to remain in a raised positionwhen the vapor velocity drops.

The elements of the bubble cap assembly can be integral or can befastened permanently or removably together. Thus, referring to FIGURES 2and 3, the cap 20 can be formed integrally with the cylindrical member23 or can be attached thereto permanently, as by welding, or removably,as by means of screw threads, slotted keys, peened lugs, etc. The use ofpeened lugs is a cheap and effective method of attaching these elements.FIGURE 3, as noted above, illustrates this type of at tachment in whichthe lugs 25 pass through slots in the cap 20 and are peened over toattach the cap to the cylindrical member 23. The flange member 26 or 44can also be integral with the cylindrical member 23 or 38, respectively,or can be attached thereto permanently,

This reduces the as by welding, or removably, as by screw threads,peened lugs, etc. Removable attachment of the cap and/or flange memberto the cylindrical member facilitates removal of the bubble cap assemblyfrom the tray if this is necessary for cleaning or repairing.

The shape of the apertures in the cylindrical member can be variedconsiderably. Vertical slots, such as slots 28 or 42 shown in FIGURES 3and 4 respectively are preferred. These slots can be trapezoidal,rectangular, triangular, oval, etc. and can have an enlarged opening atthe upper end thereof, such as large openings 30 of slots 28 in FIGURE3. The provision of enlarged opening 30 at the top of each slot makes itpossible for vapor to escape readily from the slots at low vaporvelocity when the cap is lifted only slightly above the tray. The slotspreferably should extend to or very close to the upper rim of thecylindrical member so that vapor will have a direct passage for escapewhen the cap is lifted slightly. In addition to vertical slots otherforms of apertures can be used, for example, the cylindrical member canbe provided with rows of perforations spaced uniformly about itscircumference.

Because of the uncluttered tray surface provided by our apparatus theresistance to liquid flow across the tray is low. Therefore, thegradient in liquid head across the tray is small. However, there will beat least a small dilference in liquid level across the tray and anadvantage of our apparatus is that the Weights of the bubble capassemblies can be adjusted to compensate for the gradient of liquidhead. The bubble cap assemblies on the liquid inlet side of the traywhere the liquid level is high can be fabricated so as to be of lighterweight than those on the outlet side where the liquid level is low. Byadjusting the weights of the bubble cap assemblies according to theirpositions on the tray the resistance to lifting of each bubble capassembly resulting from the weight of the assembly and the liquid headabove it can be made reasonably uniform so that all of the caps willopen at about the same vapor velocity.

Still another advantage of our novel apparatus is its simplicity andease of fabrication. FIGURE 6 shows a suitable pattern for stamping thecylindrical member from sheet metal. The two smaller sides of the flatmember with cutout portions as shown in FIGURE 6 can be brought togetherto form a cylindrical member 46 for our bubble cap assembly. The lugs 48are inserted into slots in a cap and are peened over to attach the capto the cylindrical member. The extensions 50 are bent approximatelyperpendicular to the vertical sides of the cylinder 46 to form thelateral or flange members of the bubble cap assembly. If desired, thecylindrical member 46 can be formed by stamping suitable slots andopenings 52 in tubular stock. Thus, because of the simplicity of thebubble cap assemblies of our invention, they can be made by inexpensivefabrication methods.

Obviously many modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof and therefore only such limitations should be imposed asare indicated in the appended claims.

We claim:

1. Vapor-liquid contacting apparatus comprising a tray having aplurality of generally circular vapor passages, each provided with avertically displaceable bubble cap assembly, said assembly comprising agenerally circular disc-like cap surmounting the vapor passage withwhich said assembly is associated, said disc-like cap having aconcavo-convex central portion and a flat rim, the outside diameter ofsaid rim being substantially larger than the inside diameter of thevapor passage and forming a seal against the flow of liquid into saidpassage when the cap rests on the tray floor, the undersurface of saidrim of said cap being provided with annular ridge means to prevent fiatsurface contact with said tray and thereby forming said seal, adepending cylindrical member attached to the underside of said cap andhanging through the vapor passage, said cylindrical member being hollowand open at its lower end and being provided with uniformly spacedvertical slots, said cylindrical member being provided at its lower rimwith a sufficient number of extensions curved outwardly to limit theupward movement of the bubble cap assembly, said extensions being spacedapart vertically from the undersurface of said tray when the rim of saidcap rests on the tray.

2. Vapor-liquid contacting apparatus comprising a tray having aplurality of generally circular vapor passages, each provided with avertically displaceable bubble cap assembly, said assembly comprising agenerally circular disc-like cap surmounting the vapor passage withwhich said assembly is associated, said disc-like cap having aconcavo-convex central portion and a fiat rim, the outside diameter ofsaid rim being substantially larger than the inside diameter of thevapor passage and forming a seal against the flow of liquid into saidpassage when the cap rests on the tray floor, a depending cylindricalmember attached to the underside of said cap and hanging through thevapor passage, said cylindrical member being hollow and open at itslower end and being provided with uniformly spaced vertical slots, saidcylindrical member being provided at its lower rim with a flange memberextending outwardly and generally circular to said cap, said flangemember being spaced apart vertically from the undersurface of said traywhen the rim of said cap rests on the tray, and said flange member beingnotched and provided with means to prevent flat surface contact with theundersurface of the tray.

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